!Magister Winonin:

It seems that the most evil creatures in the Devastated Castle are the *Doom Servants, Doom Guards, Doom Archers, Doom Troopers, Doom Warriors, and Doom Knights.*

Remember that you cannot fill your crystal with the souls of weaker creatures. The crystal develops a refined taste and demands souls of greater evil. If you notice that your crystal has stopped absorbing souls, seek out creatures of greater evil.